We propose to isolate the messenger RNA for an immunoglobulin light chain of known amino acid sequence from a mouse myeloma cell line and to determine large parts of its nucleotide sequence. No mammalian mRNA has yet been well characterized and the immunoglobulin mRNA is of particular biological interest. This study will provide detailed information about the end groups on the RNA, the exact sequences around the codons for initiation and termination of protein synthesis, any secondary structure in the messenger, the sequence of any untranslated regions, and the length and location of any poly-A rich sequences. This information could lead to new insights about the signals which control the synthesis and the translation of messenger RNA. Moreover, new lines of experimentation would be opened by the availability of a pure, well characterized species of mammalian mRNA. RNA highly labelled with P32 is essential for the radiochemical sequencing methods that we plan to use. Mouse myeloma cells have great advantages for labelling a specific mRNA because they grow rapidly in culture and produce large amounts of a single species of immunoglobulin. We plan to prepare polyribosomes from the microsomal fraction of myeloma cells and to isolate RNA from the polyribosomes under conditions which minimize degradation. The RNA will then be fractionated by ultracentrifugation and gel electrophoresis. The light chain mRNA will be identified either by showing that it can direct the synthesis of the light chain in a cell-free system or in Xenopus eggs or by showing that several long oligonucleotides have sequences which code for parts of the known amino acid sequence.